The present invention relates to a developer mixture of a magnetic toner and a magnetic carrier for use in electrophotographic process, electrostatic printing, electrostatic recording, etc. and an electrostatic developing method using the developer mixture. Specifically, the present invention relates to a developer mixture causing less carrier adhesion on a photosensitive surface and producing a high quality image, and an electrostatic developing method using such a developer mixture.
In a known electrophotographic imaging process and electrostatic recording process utilized in printers, facsimile machines, etc., an electrostatic latent image is formed on the surface of a cylindrical photosensitive drum. A developing roll composed of a sleeve and a permanent magnet mounted interiorly in the sleeve and rotatably relative to the sleeve is disposed opposite to the photosensitive drum. A magnetic developer is magnetically attracted on the surface of the sleeve and transported by the relative rotation of the sleeve to the magnet. The magnetic developer transported to a developing zone forms a magnetic brush which brushes the surface of the photosensitive drum to develop the electrostatic latent image to a visual toner image. The toner image is transferred onto a recording sheet which is then heated to permanently fix the toner image thereon.
Although a one-component developer only containing a toner comprising a binder resin and a magnetic powder may be used, a two-component developer which is a mixture of a magnetic carrier and a non-magnetic toner has been widely used in the magnetic brush developing method. Although the two-component magnetic developer produces images with a high density and high resolution, it is insufficient in halftone reproduction and requires a means for controlling the toner concentration in the developer. The one-component developer has problems of electric agglomeration of toner particles, developing defects due to insufficient electrification of toner particles on the sleeve.
To eliminate the above problems, a developer mixture of a magnetic carrier and a magnetic toner, which is expected to have advantageous properties of both the two-component developer and the one-component developer mentioned above, has been proposed.
To meet the recently increasing demand for producing high quality images, it has been proposed to use a magnetic toner with a reduced size, for example a magnetic toner having an average particle size of 5to 15 .mu.m. The reduction of the toner size consequently requires a reduced size of the carrier. However, the carrier of reduced size is likely to produce images with white spots (developing defects) in the image area due to the carrier adhesion. The carrier adhesion may be prevented by increasing the magnetization intensity of the carrier. However, an excessively increased magnetization makes the magnetic brush rigid to result in reducing the developability.
Japanese Patent Laid-Open No. 56-159653 discloses a developer comprising a magnetic toner and an aggregated conductive magnetic particle having a volume average particle size smaller than that of the magnetic toner. Japanese Patent Laid-Open No. 57-155553 discloses a developer comprising a carrier having a particle size of 70 to 300 .mu.m and a magnetic toner having a particle size of 5 to 50 .mu.m.
However, the developers disclosed in the above documents still suffer from occurring the carrier adhesion to fail to produce images with no white spot in the image areas. Also, the aggregated conductive magnetic particle of Japanese Patent Laid-Open No. 56-159653 has a specific volume resistance of 10.sup.9 .OMEGA..cndot.cm or less. Therefore, not only the toner but also the aggregated conductive magnetic particle, which are different from each other in color and fixing property, deposits on the latent image to fail to produce images of high quality.